DESCRIPTION: Children who experience chronic stress in early childhood are at risk for poor physical and mental health in adulthood. Early life stress shapes biological stress systems, with lifelong health consequences. Given the powerful long term implications of early life stress for public health, it is important to investigate how chronic early life stress exerts such far-reachin effects and which environmental stressors have the greatest impact on biological stress. Young children depend on sensitive caregivers to provide social buffering, protecting the developing brain from being exposed to too much of the stress hormone cortisol. However, poverty, household chaos, and parenting stress can disrupt this social buffering and may increase exposure to the stress hormone cortisol. To fully understand this process, a biological marker of chronic stress in infancy and early childhood is critically needed. Salivary cortisol, the most widely used marker of biological stress, measures acute rather than chronic stress, and salivary cortisol levels vary greatly from day to day. Cortisol is deposited in the hair shaft as it grows, o hair cortisol measures chronic stress over several months with a single sample. We and others have demonstrated the feasibility of measuring hair cortisol in young children. Yet much remains unknown about how this new method relates to established salivary cortisol methods. Salivary and hair measures may provide distinct information about the biological stress system, indexing daily regulation versus cumulative exposure. It is critical for researchers to make informed decisions about whether to use hair or salivary cortisol measures or both, as we work toward a comprehensive understanding of the processes through which early life stress influences biological stress systems. The current study will measure hair and salivary cortisol in 80 infants and 80 preschool children. Our first objective is to examine the extent to which hair cortisol and diurnal salivary cortisol measures are related in young children. Our second objective is to understand how established early life stressors relate to hair cortisol levels in young children. We will assess maternal sensitivity; maternal hair cortisol; maternal and paternal perceived stress; sleep; breastfeeding; childcare; fearfulness; household chaos; and socioeconomic status (SES) as predictors of this hair cortisol measure of chronic stress. This essential research will yield valuable data about how the innovative hair cortisol measure compares to ubiquitous salivary cortisol measures for two age cohorts, infancy and preschool, and how it relates to environmental stressors. Results will move the field forward in the quest to fully understand how early life stress shapes the development of biological stress systems. Ultimately, this knowledge will inform targeted prevention and intervention approaches to buffer young children from the biological consequences of early life stress, thus reducing long term public health burden.